https://github.com/metabrainz/musicbrainz-server/blob/master/root/components/relationships.tt#L48 specifically restricts the relationships shown under "Related series" to URL ones. This is not very useful somewhere like https://musicbrainz.org/event/3eb1d917-49a0-4529-8fbc-998809efaba2 where not even the artist for the tour is listed. I think the idea behind this was to make sure "part of" relationships weren't listed, which makes sense, but should probably be achieved by specifically blocking "part of" relationships instead.